Electrical filter



April 2, 1946.' AA. BADMAIEFF y 2,397,772'

ELECTRICAL FILTER Filed pct. ll, 1943 OUTPUT [avouer/VE) OU TPUT INPUT Snvenor aalnuule iff. Zia/JMJ# Patented Apr. 2, `1946 ELECTRICAL FILTER Alexis Badmaieif, Indianapolis, Ind., assigner -to Radio Corporation of America, a corporation of Delaware i Application october 11,1943, serialfNo. 505,800

9 Claims.

My invention relates to adjustable electrical filters and more particularly to rejection filters used in audio frequency amplifiers and transmission circuits for rejecting some particular frequency or band of frequencies.

- An object of my invention is to provide an adjustable filter for rejecting a band of frequencies of desired width with maximum attenuation adjustable at the will of the operator to any frequency within the band.

Another object of my invention is to provide a simplified and inexpensive means for controlling the characteristics of a tuned electrical rejection filter without varying. the actual tuning.

The invention will be best understood from the following description when read in the light of the accompanying drawing, in which,

Figure l is a schematic diagram of a rejection bridge. The other two arms are the two resistances R1 and Rz, in the form of a potentiometer,

R1 and Rz varying inversely with adjustment of and R: are adjusted so that their ratio is the sameas the impedances of the two tuned circuits at resonance, the signal will be cancelled out completely, leaving only the harmonics, if any. When the frequency increases yor decreases from the resonance point, the impedance of L1C1 will decrease, and the impedance IaCz will increaseand since one of the nlters will become capacitive and the other inductive. their voltages will be in Phase and will add. Since this effectv is evident only just before and lust after the resonance point (because after that the series nlters impedance is much too large as compared to that of the parallel filter). it will tend to increase the steepness of the slopes of the response curve near the shoulder-and square the shoulder. The aeneral response curveis shown inl'igure 2. An input circuit is connected to conlulate Points I and 2 of the bridge and the output circuit is connected to the other coniugate points 3 and 44 of thebridge.

the potentiometer. When the two resistances R1 Referring to Fig. 3, when the parallel filter is tuned to a lower frequency f1 than la oi' the series filter, as by adjustable means for the inductors L1 and La or for the capacitors, then both lters are capacitive between the two resonance frequencies, fi and fz, and their voltages in this bridge circuit are opposite in phase. These voltages therefore can be made to cancel by adjustmentofRi and Rz at anyl desired frequency between the two resonant frequencies f1 and fz. The advantages of this, besidesthe -convenient method of frequency rejection adjustment, is that the point of balance at f: vcan be very near the peak of the series filter, effecting a steeper slope and moreang'uiar shoulder., as shown in Figure 3. e

'I'he input circuit is connected to the end ter- 'minals I and 2 of the potentiometer RiRz and the outputvto the slider, or intermediate point,

l and a point 3 between the resonant circuits. The potentiometer comprises two non-resonant impedance elements as two arms of the bridge'. Between the resonance points of L1C1 and LaC: the system is capacitively reactive, and there is a phase shift yrange included between these points but falling short thereof, as represented by point 8 and i0', Fig. 3. Points I and 'I represent two intermediate points of rejection bands selected when the potentiometer is adjusted for balance to other frequencies than h in Fig. 3. Since the resonance points of the two tuned circuits occurs at fz and f1 respectivelythe region between `those twov resonance points is either capacitive or-y inductive, depending on which is f the lower in frequency of the two. Since in the bridge circuit, the phases Aof the two circuits are in relation, any point can be picked for balance between fi and fz. by means of adjustment of the amplitude. 'I'hi's adjustment is made by R1.

The response curve can be reversed by making the parallel filter LiCi tune to a higherv frequencyA than Loca; then both filters become inductively yreactive between their two resonant frequencies. asshown in Figure 4.

While the circuit of Figure 1 was used with I audio frequency currents, it'can also be used, with changed values. for carrier or radio frequency. currents. In somecases. however, the resistance potentiometer may be 4replaced by a capacity llo tentiometerCs. C4. as shown in Fig. 5, when using radio frequency currents. The filter circuits .LiCi and IaC: may be .tuned simultaneously by means of sansed adjustable iron cores I2 and ll,'respectively, todiiierent as y vprovement.

This rejection filter can be used wherever a single frequency is to be. rejected, as for instance 120 cycle hum in amplifier or transmission lines. In that case both tuned circuits are tuned to that frequency. If, however, the frefluency to be rejected shifts within two limits, the potentiometer is a convenient means to tune the unwanted frequency out when the two resonant circuits are tuned to the respective two limits.V

When used in conjunction with another band passlter or a high or low pass filter, it is often necessary to reject a frequency very close to 'the pass frequency. lin that case either an inductive or capacitative spread is used, and the bale combined characteristic of said resonant circuits between the resonant points of saidcircuits.

3. In an electrical wave filter network', a bridge comprising a parallel resonant circuit in one arm tuned to one frequency, series resonant circuit in another arm tuned toa different frequency,

' tween the resonant points of said circuits.

ance is attained by the potentiometer, to reject y that frequency.

-Various circuits and combinations thereof embodying the invention have been disclosed. Numerous changes could be made in the specific circuits by those vsl-rilled in the artwithout departing from the spirit of the invention.

I claim as my invention:

1. In an electrical wave filter, a bridge comprising a parallel resonant circuit in one arm, a series resonant circuit in an adjacent arm, said circuits being tuned to different frequencies, and A a potentiometer connected to form the two other arms of said bridge, an input circuit connected 4. The invention asset forth in claim 3 wherein capacitative characteristic.

'7. An electrical filter network comprising in combination, an input and an output circuit, a parallel resonant circuit tuned to one'frequency, a series resonant circuit tuned to a diiferent frequency, and a potentiometer connected between said resonant circuits, and a connection between a point on said potentiometer and said output circuit for adjusting the maximum attenuation eect of said filter between saidv frequencies.

8. The invention as set forth in claim 7 wherein means is provided for simultaneously varying the tuning of said resonant circuits.

9. An electrical bridge filter comprising in combination, input and output circuits connected respectively to conjugate points of saidbridge, one arm of said bridge including an adjustable resonant circuit tuned to one frequency, another 

